The invention relates to line drivers, in particular to a driver for the LIN-bus (Local Interconnect Network). The LIN-bus is used in automotive applications and is a concept of a single master and multiple slaves connected to a single bus wire. The bus wire is connected to a supply voltage which is positive with respect to ground by means of an external pull-up resistor which keeps the bus wire in a recessive voltage level corresponding to a relatively high positive voltage near to the positive supply voltage. The master and the slaves have a built-in line driver for pulling down the bus wire voltage to a dominant voltage level corresponding to a relatively low positive voltage near to ground level in response to a data signal.
The EMI (Electro Magnetic Immunity) performance of a LIN bus driver used in an automotive environment is a very important parameter. RF (Radio Frequency) disturbances on the line driver output can result in misinterpretation of the recessive and dominant voltage levels, or can result in an asymmetrical propagation delay between the rising and falling edges of the LIN-bus signals, which causes a duty cycle different from the desired 50%. Both can be a cause for faulty communication.
A current source output stage is a good choice with respect to EMI. Such a current source output stage allows the RF disturbances to be superimposed on the original LIN-bus signal. The RF disturbances can be filtered out by means of low-pass filters at the input of the receivers. However, a current source output stage causes an unwanted delay during a transition in the bus signal from ground level to positive supply voltage level. When using a current source to control the voltage on the LIN-bus line, not only the current from the current source, but also the load impedance on the bus line dictates the LIN-bus voltage. According to the LIN protocol specification the load may vary between 500 Ohm and 1 kOhm.
In case of a 1 kOhm load a delay occurs in the transition from ground level to the positive supply voltage level, because the ramp generator which drives the current source output stage is designed for the worst-case load of 500 Ohm. In case of a 1 kOhm load the current source will try to pull down the LIN-bus voltage below ground level when the drive signal from the ramp generator crosses a certain level, which is not possible. When the direction of the output current is reversed, the LIN-bus voltage remains at ground level during a certain waiting time until the ramp generator again crosses said level in reverse direction. This waiting time causes a delay between the on-set of the transition from ground level to the positive supply voltage level and the corresponding edge of the data signal.
It is an object of the invention to provide a line driver which is less sensitive to load variations. According to the invention this object is achieved by a line driver comprising:
a driver output terminal for connection to a line to be driven;
a reference terminal for connection to a reference voltage;
a ramp generator for providing a substantially linearly inclining and declining ramp signal in response to a data signal, the ramp generator having a control input for connecting a control signal, the ramp signal having a slew rate substantially proportional to the control signal;
a current source output transistor having its first main electrode coupled to the reference terminal and its second main electrode coupled to the driver output terminal for providing a current between the driver output terminal and the reference terminal which current is proportional to the ramp signal;
a comparator for comparing a voltage drop across the current source output transistor to a threshold voltage; and
the comparator having an output coupled to the control input of the ramp generator and being operable to increase the slew rate of the ramp signal when the voltage drop is smaller than the threshold voltage.
The comparator considerably increases the slew rate as soon as the voltage across the current source output transistor drops below the threshold voltage, as a result of which the output signal of the ramp generator and thus also the output current of the line driver quickly reaches its end value. In the reverse direction, when the voltage across the current source output transistor is still below the threshold voltage, the output current of the line driver quickly decreases with high slew rate until the voltage across the current source output transistor exceeds the threshold voltage thereby considerably reducing the waiting time.
In the embodiment as claimed in claim 2 the control signal is a current source and the comparator is a simple transistor. The current from the transistor is added to the current from the current source to increase the slew rate of the ramp signal.
In the embodiment as claimed in claim 3, the current from the transistor is conveniently added to the current from the current source by means of a current mirror.
The embodiment as claimed in claim 4 provides a driver stage for driving the current source output transistor from a controllable current source. The reference transistor and the current source output transistor are scaled copies of each other. Therefore, the current flowing through the current source output transistor is proportional to the current flowing through the reference transistor. The resistor provides a low-impedance path to ground for RF disturbances reaching the control electrode of the current source output transistor through the capacitance between the second main electrode and the control electrode of the current source output transistor and prevents these disturbances from reaching the driver stage.